1. Technical Field
This invention relates to the technology for eliminating hinged drill caps resulting from through-hole drilling of at least moderately ductile metals, and particularly to technology that eliminates drill caps resulting from using drill bits with reduced width chisel edges or split points.
2. Discussion of the Prior Art
When using reduced width chisel edged drills or split point drills for through-hole drilling (drilling through the entire depth of the workpiece) in ductile steels, drill caps are commonly formed as the drill exits the workpiece. Drill caps are thin sections of material, resembling the shape of the drill point, which are extruded rather than cut during the drilling process. Drill caps form as the thrust force in drilling overcomes the material's resistance to axial plastic flow. The thickness of the drill cap is a function of the drilling feed, and is typically in the range of 0.003-0.006 inches. As the drill exits the workpiece, the drill cap begins to break free from the workpiece around part of the perimeter of the hole at one side thereof, and then folds over and hinges along an edge of the drilled hole at the other part of the perimeter.
Drill caps can be a problem for many powertrain drilling operations of automotive powertrain components, such as clutch plate bearing sleeves in automatic transmissions, requiring very small drill openings for lubrication purposes. Split point style drills are often used in production drilling operations. Using such drill design, when drilling steel components, frequently leads to the formation of drill caps which remain attached to the part. Drill cap formation itself is not a concern, but the retention on the part can be a severe problem. They are difficult to remove with automated deburring equipment. Automated deburring may involve: blowing of abrasive grit to knock off the drill caps, such method being relatively by expensive; wire brushing which more frequently tends to push the drill cap back up into the hole, closing such hole; or electrolytic erosion by immersing the workpiece in a bath which often allows an electric arc to occur between the surface of the workpiece and the hinged cap thereby avoiding cap removal.
If the drill caps are not removed they can adversely affect subsequent machining (such as damage flow honing tool), affect subsequent hardening (material handling equipment having difficulty of inserting folders into the workpiece with the presence of hinged drill caps), or assembly processes. If drill caps are present after final assembly, their presence during system operation (such as in a transmission or in an engine) may lead to improper performance or warranty repair.
Published prior art has not solved the problem of hinged drill caps. Drilled workpieces have been bathed or flushed with a cooling medium for loose chip removal, or subjected to a suction to pick up loose debris; these techniques do not avoid the formation of hinged drill caps. Coolant jets have been directed at the shear edge cutting zone in some drilling operations, but this also fails to prevent hinged drill caps.